PTERIDOPHYTA 387 
of a simple siphonostele, but is fre- 
quently called a polystele and may be 
further complicated by the formation of 
extra bundles which develop as branches 
from the original cylinder. 
In primitive siphonosteles the xylem 
is bordered by phloém, both toward the 
center and toward the exterior (Fig. 
427). In certain more advanced types 
there is no internal phloém. The steles 
of modern conifers and dicotyledons 
appear to have been derived from a 
siphonostele from which the internal 


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Fic. 428. Diagram of a si- 
phonostele of a fern, show- 
ing leaf gaps where leaf 
traces leave the stele 
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Fic. 429. Cross section of stele of a fern (Cyclophorus adnascens), in which 
leaf gaps are so long that there appear to be five separate bundles in the stele 
Note the sclerenchyma in the center of the stele and also the cylinder of scle- 
renchyma near the outside of the cortex. (x 50) 
e 
